1. Field of the Invention
The present invention relates to a surface potential electrometer for measuring the surface potential of a recording member by chopper conversion thereof into an AC current and also to an image forming apparatus such as a copier capable of controlling the image formation by means of said surface potential electrometer.
2. Description of the Prior Art
In the field of electrophotographic apparatus there is already known non-contact measurement of the surface potential of the photosensitive member for suitably controlling the process means in order to achieve stable image formation. As a method of such non-contact surface potential measurement, the present applicant already proposed, in the U.S. patent application Ser. No. 68,416, a surface potential electrometer in which a cage-shaped rotor is rotated around a detecting electrode for chopping the lines of electric force thereby detecting the surface potential as an AC signal. Such electrometer requires a motor for rotating said cage rotor, and such motor has to be small and of high precision in order to avoid the effect of motor rotation on the output signal, thus leading to a high manufacturing cost. Also such electrometer requires a high precision in the mounting in order to avoid eventual vibration of the device including the electrometer since the detection output signal is dependent on the distance between the surface to be measured and the measuring electrode or the chopping rotor.